Retinoic acid-related nuclear receptor gamma t (RORγt) is a nuclear receptor, exclusively expressed in cells of the immune system, and a key transcription factor driving Th17 cell differentiation. Th17 cells are a subset of CD4+ T cells, expressing CCR6 on their surface to mediate their migration to sites of inflammation, and dependent on IL-23 stimulation, through the IL-23 receptor, for their maintenance and expansion. Th17 cells produce several proinflammatory cytokines including IL-17A, IL-17F, IL-21, and IL-22 (Korn, T., E. Bettelli, et al. (2009). “IL-17 and Th17 Cells.” Annu Rev Immunol 27: 485-517.), which stimulate tissue cells to produce a panel of inflammatory chemokines, cytokines and metalloproteases, and promote recruitment of granulocytes (Kolls, J. K. and A. Linden (2004). “Interleukin-17 family members and inflammation.” Immunity 21(4): 467-76; Stamp, L. K., M. J. James, et al. (2004). “Interleukin-17: the missing link between T-cell accumulation and effector cell actions in rheumatoid arthritis” Immunol Cell Biol 82(1): 1-9). Th17 cells have been shown to be the major pathogenic population in several models of autoimmune inflammation, including collagen-induced arthritis (CIA) and experimental autoimmune encephalomyelitis (EAE) (Dong, C. (2006). “Diversification of T-helper-cell lineages: finding the family root of IL-17-producing cells.” Nat Rev Immunol 6(4): 329-33; McKenzie, B. S., R. A. Kastelein, et al. (2006). “Understanding the IL-23-IL-17 immune pathway.” Trends Immunol 27(1): 17-23.). RORγt-deficient mice are healthy and reproduce normally, but have shown impaired Th17 cell differentiation in vitro, a significantly reduced Th17 cell population in vivo, and decreased susceptibility to EAE (Ivanov, II, B. S. McKenzie, et al. (2006). “The orphan nuclear receptor RORgammat directs the differentiation program of proinflammatory IL-17+ T helper cells.” Cell 126(6): 1121-33.). Mice deficient for IL-23, a cytokine required for Th17 cell survival, fail to produce Th17 cells and are resistant to EAE, CIA, and inflammatory bowel disease (IBD) (Cua, D. J., J. Sherlock, et al. (2003). “Interleukin-23 rather than interleukin-12 is the critical cytokine for autoimmune inflammation of the brain.” Nature 421(6924): 744-8. ; Langrish, C. L., Y. Chen, et al. (2005). “IL-23 drives a pathogenic T cell population that induces autoimmune inflammation.” J Exp Med 201(2): 233-40; Yen, D., J. Cheung, et al. (2006). “IL-23 is essential for T cell-mediated colitis and promotes inflammation via IL-17 and IL-6.” J Clin Invest 116(5): 1310-6.). Consistent with these findings, an anti-IL23-specific monoclonal antibody blocks development of psoriasis-like inflammation in a murine disease model (Tonel, G., C. Conrad, et al. “Cutting edge: A critical functional role for IL-23 in psoriasis.” J Immunol 185(10): 5688-91).
In humans, a number of observations support the role of the IL-23/Th17 pathway in the pathogenesis of inflammatory diseases. IL-17, the key cytokine produced by Th17 cells, is expressed at elevated levels in a variety of allergic and autoimmune diseases (Barczyk, A., W. Pierzchala, et al. (2003). “Interleukin-17 in sputum correlates with airway hyperresponsiveness to methacholine.” Respir Med 97(6): 726-33.; Fujino, S., A. Andoh, et al. (2003). “Increased expression of interleukin 17 in inflammatory bowel disease.” Gut 52(1): 65-70; Lock, C., G. Hermans, et al. (2002). “Gene-microarray analysis of multiple sclerosis lesions yields new targets validated in autoimmune encephalomyelitis.” Nat Med 8(5): 500-8.; Krueger, J. G., S. Fretzin, et al. “IL-17A is essential for cell activation and inflammatory gene circuits in subjects with psoriasis.” J Allergy Clin Immunol 130(1): 145-154 e9.). Furthermore, human genetic studies have shown association of polymorphisms in the genes for Th17 cell-surface receptors, IL-23R and CCR6, with susceptibility to IBD, multiple sclerosis (MS), rheumatoid arthritis (RA) and psoriasis (Gazouli, M., I. Pachoula, et al. “NOD2/CARD15, ATG16L1 and IL23R gene polymorphisms and childhood-onset of Crohn's disease.” World J Gastroenterol 16(14): 1753-8, Nunez, C., B. Dema, et al. (2008). “IL23R: a susceptibility locus for celiac disease and multiple sclerosis?” Genes Immun 9(4): 289-93; Bowes, J. and A. Barton “The genetics of psoriatic arthritis: lessons from genome-wide association studies.” Discov Med 10(52): 177-83; Kochi, Y., Y. Okada, et al. “A regulatory variant in CCR6 is associated with rheumatoid arthritis susceptibility.” Nat Genet 42(6): 515-9.). Ustekinumab (Stelara®), an anti-p40 monoclonal antibody blocking both IL-12 and IL-23, is approved for the treatment of adult patients (18 years or older), with moderate to severe plaque psoriasis, who are candidates for phototherapy or systemic therapy. Currently, monoclonal antibodies specifically targeting only IL-23, to more selectively inhibit the Th17 subset, are also in clinical development for psoriasis (Garber K. (2011). “Psoriasis: from bed to bench and back” Nat Biotech 29, 563-566), further implicating the important role of the IL-23- and RORγt-driven Th17 pathway in this disease. Results from recent phase II clinical studies strongly support this hypothesis, as anti-IL-17 receptor and anti-IL-17 therapeutic antibodies both demonstrated high levels of efficacy in patients with chronic psoriasis (Papp, K. A., “Brodalumab, an anti-interleukin-17-receptor antibody for psoriasis.” N Engl J Med 2012 366(13): 1181-9; Leonardi, C., R. Matheson, et al. “Anti-interleukin-17 monoclonal antibody ixekizumab in chronic plaque psoriasis.” N Engl J Med 366(13): 1190-9.). Anti-IL-17 antibodies have also demonstrated clinically relevant responses in early trials in RA and uveitis (Hueber, W., Patel, D. D., Dryja, T., Wright, A. M., Koroleva, I., Bruin, G., Antoni, C., Draelos, Z., Gold, M. H., Durez, P., Tak, P. P., Gomez-Reino, J. J., Foster, C. S., Kim, R. Y., Samson, C. M., Falk, N. S., Chu, D. S., Callanan, D., Nguyen, Q. D., Rose, K., Haider, A., Di Padova, F. (2010) Effects of AIN457, a fully human antibody to interleukin-17A, on psoriasis, rheumatoid arthritis, and uveitis. Sci Transl Med 2, 5272.).
All the above evidence supports inhibition of the Th17 pathway by modulating RORγt activity as an effective strategy for the treatment of immune-mediated inflammatory diseases.